1. Field of the Invention
The present invention relates to the technical field concerned with the production of hydraulic, pneumatic, or pneumatic-hydraulic tools.
In particular, the present invention concerns a rivet gun operated by pneumatic-hydraulic means.
The rivet gun is designed for application of rivets provided with an internal thread.
2. Prior Art
It is known that rivets are usually fixed to laminate structures, basically including pieces of rigid sheet made from metal or other suitable materials.
Suitable tools, preferably operated by pneumatic or pneumatic-hydraulic means, are used for fixing the rivets to the laminate structure. These tools take usually the shape of a gun, so that they can be easily handled by an operator who has to apply the rivets to the internal thread.
Among various constructive and operative configurations, the one which uses pneumatic-hydraulic means has resulted to be the most effective, reliable and cheap.
Basically, known rivet guns include each one a hollow tool body, symmetrical with respect to a longitudinal axis. A handle made integral with the body, extends downwards therefrom.
The body of the tool features inside and in the fore part, a cylindrical channel having variable diameter and, in the rear part, a cylindrical chamber. The chamber includes a reversible pneumatic engine, that is connected to a stem, named in the following as rivet holding stem, that goes outside in the region of the head of the rivet gun.
The rivet holding stem is therefore rotated by the motor. The rivet holding stem is also threaded along the portion protruding outwards from the rivet gun head.
The tip portion of the rivet holding stem can be replaced, when needed, with other similar portions having different diameters, to mount rivets of different diameters.
The pneumatic engine is driven into rotation, usually in clockwise direction, by a blow of compressed air which is supplied through an input duct. A suitable push button allows or cuts off the blow of air.
The compressed air to be discharged goes out of the engine, with a lower pressure, via a discharge duct and, partially, through intermediate discharge holes.
Reverse rotation of the engine is obtained by means of a change-over switching device, that is also located in the handle. When operated, the change-over switching device closes the input duct and deviates the blow of air to the discharge duct. The discharge duct, in reversed condition, works as a supply duct. Discharge to the outer environment takes place by passing the discharge air through the clearances always present in the connection regions of the various components of the pneumatic engine.
It is clear enough that, with such constructive configuration, reverse rotation, or counter-clockwise rotation, of the pneumatic engine cannot be efficient and the resulting torque is rather small.
The pneumatic engine-rivet holding stem assembly may also move axially backwards, against a spring which normally keeps it in an advanced position. The stroke of the assembly motion is suitably delimited by stop surfaces.
The axial motion of the pneumatic engine-rivet holding stem assembly is determined by a hydraulic system, that includes an expansible chamber supplied with oil under pressure coming from an hydraulic cylinder via an input duct.
The hydraulic cylinder is located in the upper part of the handle of the rivet gun. The expansible chamber is located in front of the pneumatic engine in the rear chamber of the rivet gun.
The hydraulic cylinder is in turn operated by a pneumatic cylinder, that has a wider cross section and is located in the lower part of the handle. The pneumatic cylinder is supplied with air under pressure coming from the same source which feeds the pneumatic engine, via suitable ducts.
The pneumatic cylinder is operated by means of a second push button located in the front part of the handle. The second push button operates a valve, that allows air under pressure to enter the pneumatic cylinder.
In this case, the piston of the pneumatic cylinder goes up, and the stem of the piston pushes upwards the piston of the hydraulic cylinder. In fact, the stem of the pneumatic piston forms the piston of the hydraulic cylinder located thereabove. The oil under pressure in the hydraulic piston is moved to the expansible chamber, that in turn moves the pneumatic engine-rivet holding stem assembly backwards.
Basically, the system including the pneumatic cylinder and the hydraulic cylinder forms a pressure booster, that permits to apply a very strong backward force to the pneumatic engine-rivet holding stem assembly while moving it backwards.
Operation of the known rivet gun described hereinabove, to apply an internal thread rivet to a laminate structure, takes place as follows:
a rivet having internal diameter and thread corresponding to those of the rivet holding stem, is set on the tip of the latter; PA0 the pneumatic engine is operated with direct rotation (clockwise), so that the rivet holding stem is also rotated and the rivet is screwed on the threaded tip of the rivet holding stem; PA0 then the rivet is placed into a corresponding hole made in the laminate structure and in abutment against a frontal surface thereof; PA0 backward motion of the pneumatic engine-rivet holding stem assembly is performed very quickly and with very big force, as previously described, and the intermediate portion of the rivet protruding beyond the hole of the laminate structure is buckled against the backside of the structure, so that the rivet is fixed; PA0 lastly, the reverse operation push button is activated for reverse rotation of the pneumatic engine, so that the rivet holding stem is unscrewed from the rivet. PA0 an elongated casing featuring inside a rear cavity and a fore channel, substantially cylindrical, aligned with said rear cavity along a longitudinal axis, with said fore channel connected to said rear cavity and opened outside in the region of a fore end of said casing; PA0 at least one pneumatic motor, housed axially inside said rear cavity; PA0 at least one segmented stem situated in said fore channel in succession with said pneumatic motor and axially connected with an output shaft of said motor, with a threaded terminal portion of said stem going out from said fore end for receiving a rivet internally threaded, said pneumatic motor and segmented stem sliding axially and in opposite directions inside said rear cavity and fore channel, against first elastic means; PA0 at least one hollow handle extending from a lower side of said casing and forming, in its lower part, at least one pneumatic cylinder, and its upper part, at least one hydraulic cylinder operated by said pneumatic cylinder and aimed at imposing said pneumatic motor and segmented stem axial sliding; PA0 said pneumatic-hydraulic rivet gun being characterized in that it includes, inside said rear cavity, a change-over switching device, connected to an input duct of said pneumatic motor via at least on pneumatic supply duct, and to a discharge duct of said pneumatic motor via at least one pneumatic discharge duct, said change-over switching device being provided for supplying, said pneumatic motor via said supply duct with a flow of compressed air coming from an infeed duct, during the direct rotation and in accordance with first control means, while discharging said compressed air via said discharge duct, and being provided for supplying said pneumatic motor via said discharge duct with a flow of compressed air coming from said infeed duct during reverse rotation and in accordance with second control means, while discharging said compressed air via said supply duct.
The rivet guns like the one described hereinabove, have some drawbacks which make their use difficult and scarcely efficient.
First of all, this technique used to reverse the rotation of the pneumatic engine makes it poorly efficient right when a very high torque would be necessary, i.e. when the tip of the rivet holding stem must be extracted from the rivet. In fact, when the rivet is buckled, the internal thread becomes damaged and anyway does no longer extend in a perfect line. Therefore, to extract the rivet holding stem from the rivet the torque exerted thereon must be higher than the one applied during the screwing step.
There are also known rivet guns in which, to overcome this serious inconvenience, the pneumatic engine is operated in reverse rotation when the stem is screwed into the rivet, and then the engine is operated with direct rotation when the stem must be extracted from the rivet. This solution actually improves the effectiveness of the rivet gun, but the screwing step becomes often difficult and slow, so that the problem cannot be said to be completely solved.
Another problem encountered with the rivet gun like the one described above, is that only the extension of the stroke of the stem can be adjusted and varied in relation to different operation conditions. In other words, when the stems has moved to cover a pre-established stroke, the rivet gun is deactivated. On the contrary, the actuating pressure cannot be adjusted. This lack of adjustment possibility for the rivet gun, provokes a risk of subjecting the rivets to excessive traction force or, in the opposite case, the rivets though buckled, do not have an adequate traction force.
A further problem which can be found in the rivet guns of this kind, derives from the fact that the controls for operation of the various steps are located separately and in different parts of the handle. This fact renders more difficult the work of an operator, in particular when the rivets must be applied to positions which cannot be easily reached.
Document EP-A-0 325 699 relates to a hydropneumatic gun for setting blind-rivet nuts, in which an air piston fitted in an air cylinder is moved to pressurize oil housed in the gun body, causing an oil piston to be retracted, so that a screw mandrel attached to the oil piston at its tip is retracted to the inner part of the gun body, thereby to exert a deforming force to the sleeve of a nut threadedly mounted on the screw mandrel. The hydropneumatic gun further comprises an air motor to be rotated by com pressed air, an air motor driving air guide passage, an air motor forward/reverse changeover mechanism for switching the rotation direction of the air motor, and a power transmission mechanism for transmitting an air motor riving force to the screw mandrel. A series of operations of the screw mandrel such as forward rotation, stop of the rotation, retraction, reverse rotation and advancement can thus be carried out smoothly and sequentially. An air motor driving air guide passage is provided between the air motor and a compressed air supply port in the gun body, while a power transmission mechanism transmits an air motor forward/reverse rotation from the air motor to the screw mandrel.
An air piston moving air guide passage is provided between the compressed air supply port and an air guide hole in the air cylinder at the air piston moving side, while a spool is slidably fitted in a communication hole communicating with the air piston moving air guide passage for opening and closing the air piston moving air guide passage. The spool is moved by a spool controlling air guide chamber between the communication hole and the compressed air supply port, in such direction as to close the air piston moving air guide passage.
A discharge passage is provided between the air guide chamber and a compressed air discharge port in the vicinity of the power transmission mechanism, in the gun body, for discharging compressed air guided in the air guide chamber, while a clutch of the power transmission mechanism is disposed in the discharge passage as a member for opening and closing the discharge passage, that is adapted to be opened when the clutch is rotated to a predetermined angle position by predetermined turning torque.